The deep ocean

The seas cover 71% of the earth's surface, 65% is open
ocean - an immense ecosystem if you just consider the surface. The vertical
extent of the open oceans, however gives this ecosystem a third dimension.
The oceans have a maximum vertical extent of almost 11km and about 80%
of the oceans is deeper than 1000m - an immense area.

We divide the oceanic environment into several main
realms or zones. The epipelagic realm (from
the surface to 200m) marks the limit of the photic zone, where daylight
is adequate for photosynthesis. In the mesopelagic
realm (from 200 to 1000m) light from the surface, though very dim, is
still visible, if the water is clear. The bathypelagic
(from 1000 to 6000m) includes the abyssal
plains but excludes deep trenches (the hadal
realm).

Deep sea fish are the species of fish that live in the
mesopelagic, below the photic zone of the ocean. The conditions here are
extreme. The water is cold - it has a constant temperature of only 2 degrees
Celsius, there is little difference between day and night and the pressure
is enormous. Animals living here have developed many strategies to cope
with that.

Because these fish live in a region with little or no
natural illumination, they cannot solely rely on their eyesight for locating
prey or their mates. Instead many deep sea fish are luminous, others have
extremely large dark-adapted eyes or long feelers to help them locate
prey in the dark of the deep ocean. Some animals don't have functioning
eyes anymore but have developed special organs that can "see"
infrared, so they can sense body heat.

Some of the most interesting fishes living in the deep
ocean (mesopelagic and bathypelagic realm) are the deep-sea anglerfishes.

Deep sea anglerfishes, habitat
and range

Eleven families of the order Lophiiformes (anglerfishes)
live in the deep sea, with nearly a hundred species, more than any other
group of bathypelagic fishes.

There are two more families of anglerfishes which live
in deep waters, Chaunacidae (Sea toads) and Lophiidae (Goosefishes). A
third family, Ogcocephalidae (Batfishes) lives mostly in shallow water
but can also be found in deep water. In contrast species of the family
Antennariidae (frogfishes)
live mostly in shallow water, close to coral reefs.

Appearance

Female deep sea anglerfishes have a globular shape, not
adapted for sustained rapid swimming, but designed for remaining motionless
much of the time. Only Gigantactinidae (whipnose anglers) have an elongate
form that is suitable for brief bursts of swimming. Unlike the colorful
frogfishes that live in coral reefs, deep-sea anglerfish skin is grimy
black or gray and fragile, without scales. In fact, their skin is so thin
it can slip off their bodies when touched by human hands retrieving them
from deep-ocean nets! They have flabby muscles and weak skeletons. Gill
slits are very small, C-shaped and placed below the pectorals and a little
behind them.

Whipnose Seadevil (Gigantactis) of the family Gigantactinidae
Copyright Danté
Fenolio

Whipnose Seadevil (Gigantactis) of the family Gigantactinidae
Copyright Danté
Fenolio

Gigantactis macronema (Family Gigantactinidae
or Whipnose anglers) with a more elongate body and a long whip-like lure
attached to the tip of the snout. In some cases the lure can be four or
five times longer than the fish itself.

Cryptopasaras couesi (triplewart sea devil) belongs to
the family of Ceratiidae. The female has three
luminescent sacs, called caruncles (A), in front of her dorsal fin. These
are a pair of low, fleshy appendages and have been interpreted as vestiges
of the first dorsal fin, for each of them encloses a spine that can be
felt if not seen. Their exact function is not known, presumably they also
aid in enticing prey to within striking distance.
The female seadevils are the largest deepsea anglerfishes, reaching up
to one and a half meters and then weighting around 10 kg, compared with
them the males are dwarfs, weighting only about 150 grams.

Bufoceratias wedli of the family Diceratiidae
(Double anglers) with two lures visible on the back, a smaller behind
the larger one

Chaenophryne melanorhabdus of the family Oneirodidae
(Dreamers) with its illicium (lure) which is sharply bent and can be positioned
directly in front of the mouth.

Lasiognathus saccostoma of the family Thaumatichthyidae
(Wolftrap anglers) with a large overbite with huge movable teeth and a
lure with three bony hooks

Lophiidae (Goosefishes) - Ogcocephalidae
(Batfishes) - Chaunacidae (Sea toads)
These anglerfishes that live on the bottom (benthic) still have fins that
look like legs, similar to the frogfishes.
Look at this website
for more information about batfishes.

Red lipped batfish - Ogcocephalus darwini
Photo Eddy Thys

Red lipped batfish - Ogcocephalus darwini
Photo Eddy Thys

Luring with bioluminescence

In open water food is much more reliably available than
in the deep ocean where meals are rare and unpredictable. Animals livingin
the seed sea have developed strategies to find food and ways to eat whatever
food there may be with as little effort as possible. In deeper waters
of the ocean there are a lot of animals, which prefer to await for the
arrival of prey or food particles rather than to search them out actively.
Many animals are not very selective in their diet and they can cope with
amazing large prey.

Female anglerfishes are classic lurk-and-lure predators.
They have a large mouth and their teeth are huge compared to their body
size. Instead of searching actively for prey, they wave a luminous lure
to attract them. They can cope with very large prey, because their expandable
jaws open double wide and they posses an elastic stomach and they eat
any prey available. But their most amazing feature is a bioluminescent
lure.

Phyllorhinichthys micractis (Oneirodidae - Dreamers)
with lure and esca which has hair shaped appendices and a long pipe with
a lens at the tip. There is a bulb like light organ (A) containing a gland
with luminous bacteria.

The lures of the female deep sea anglerfish (male anglerfish
have no luminous organs) is much more elaborate than the lure of the shallow
water species and in addition bioluminescence is used to attract the prey
in the dark. The lures are modified dorsal fins and contain luminous bacteria
which produce a bluish, greenish light. The light and movement of the
lure attract the prey to within reach of the gaping jaws. The anglerfish
Cryptopsaras can slide the rod part of the apparatus back into a groove,
drawing the lure and prey closer to the mouth. It can rotate the lure
tip and produce a flash from it as well as a glow. All this is controlled
by enzymes and the contraction of chromatophores.

The lures of deep sea anglerfishes are extraordinary
elaborate, with sensory filaments, papillae, light pipes and shutters.
It may be, that different species mimic different kinds of small prey
like shrimps to attract larger prey. One anglerfish (Caulophryne) has
a lure ornamented with many filaments but it is not luminous. An other
(Linophryne) has not only a luminous lure on the head but also a multibranched
barbel hanging from the lower jay. The barbel filaments contain many more
bioluminescent organs.

Linophrynidae (Leftvents)
with lure and barbels. The Latin name literally means "toad that
fishes with a net". Linophryne algibarbata (left) and Linophryne
arborifera (right).

The lure on this anglerfish has a short rod attaching it to the
fish, so the glowing bulb sits on the face of the fish.
Copyright Danté
Fenolio

Some deep sea anglerfish have positioned their lure inside
the mouth. The wolftrapangler Thaumatichthys axeli is a sit-and-wait ambush
predator with a luminescent lure hanging from the roof of his overshot
mouth, which is fringed with hooked sharp teeth. This anglerfish hovers
just above the substrate (not sitting on it).

Other sense organs

Some deep sea anglerfishes have so called neuromast structures
or organs formed by groups of hair cells. The structures look like antennae
or thick hair and sit on the trunk or are mounted on long stalks or fin
rays. With these freestanding structures they can sense the "noise"
engendered by a fish's movement. While floating neutrally buoyant in the
water, the anglerfish can probably sense the direction and the velocity
of approaching prey.

Caulophryne polynema of the family Caulophrynidae
(Fanfins) relies solely on its superficial sensory neuromast structures
to find prey and doesn't have a luminous lure. It also has soft skin and
a highly expandable stomach.

Reproduction

With such small numbers of animals spread very thinly
over an enormous are, it can be difficult to find a mate.

Deep sea anglerfish males reach sexual maturity soon
after metamorphosis (change from larval stage to adult) but females take
much longer. One estimates that there are 15 to 30 ripe males to every
female anglerfish, probably it is very difficult for anglerfishes to find
a partner in the deep sea. In most species the males may attach briefly
during mating but the deep-sea anglerfish (Families Ceratiidae, Caulophrynidae,
Photocorynidae, Linophrynidae and Melanocetidae) shows a very strange
sexual dimorphism (differences between males and females).

In most species the female deep sea anglerfish is much
larger than the male anglerfish, actually they are real dwarfs compared
with their mate. For example the largest females of the genus Gigantactis
grow to 40cm in length, whereas the largest males only grow to 2cm. Unlike
their sluggish partners they have muscular bodies for active swimming.
For many years fish biologists were very confused by these differences
and misidentified male and female anglers as completely different species.

When they are born, male anglerfishes have no other role
than to search out a female. The male has no bioluminescent lure, probably
they never even feed. Instead they have larger eyes, presumably for spotting
the females flashing lure and a much larger olfactory organ in front of
their eyes. This difference in size is probably, because in deep sea sexual
pheromones are very important in mate location. Since mature males of
anglerfishes greatly outnumber mature females, so heightened sensitivity
to female pheromones is an advantage.

After locating a female anglerfish, the small male attaches
himself to her body by biting into her belly. His teeth and the jaw recedes,
the skin fuses and the blood systems of both animals merge. Effectively
the male becomes parasitic and doesn't have to find food by himself anymore,
actually the intestine regresses, since it is not used anymore. In fact,
about the only important internal organ is a large testis. The female
has a guaranteed supply of sperms and the male is taken care of the rest
of his life. Presumably the female anglerfish even controls the delivery
of sperms with her hormones so it is coordinated with the moment she ejects
her eggs from the body. Once fertilized the eggs which contain large oil
droplets to give them buoyancy float to the surface of the ocean.

Neoceratiidae (Toothed seadevils)
with attached male anglerfish. Females may occasionally even have more
than one attached male.

Details of the male anglerfish attached to female by
two outgrowths from the front of the head, that are fused at the tip.

Defensive behavior

The best way to avoid being eaten is to avoid being seen
or sensed. Deep sea anglerfishes usually are of dark color and remain
floating nearly motionless while sensing approaching prey and also avoiding
being noticed by other predators. It is not known if they have other defense
mechanisms such as venom. Bioluminescence can also serve as a decoy, for
example the luminescent barbels of the Linophrynidae (Leftvents) hanging
beneath the body might confuse possible predators.

Photos

I can't offer you any of my own
photos, since I have never seen a live deep sea anglerfish. My photos
are about frogfishes living on coral reefs. Please
check out the following pages:

Fishbase has the largest collection of photos of deep
sea anglerfishes - look at the taxonomy
page and click on the link next to the species names.

The Australian
Museum has a website with several photos and a good description and
some close-ups of the lure, esca and caruncles. Search by Latin name.

NOVA
has a page about deepsea anglerfishes with some nice photos of living
anglerfishes taken underwater.

Google image search works
quite well, if you search for the Latin names.

Some really interesting black-and-white photos and illustrations
I have found on Tierlexikon,
a German page. Even if you don't understand the text, just hover with
your cursor over the text that says "(Abbildung ...)" and a
small photo is visible. Then click on the underlined word to see a larger
photo.